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ONLINE COVER A Gateway into the Brain. Brain tissue of a germ-free adult mouse (left) takes up more of the radioligand 11C-raclopride during PET imaging (yellow) compared to brain tissue of a pathogen-free mouse (middle) and a germ-free mouse colonized with a gut microbiota (right). Increased radioligand uptake by the germ-free mouse brain indicates a more permeable blood-brain barrier (BBB); the presence of a gut microbiota renders the BBB less permeable leading to reduced radioligand uptake (red) Braniste et al.. [CREDIT: MIKLÓS TÓTH, KAROLINSKA INSTITUTE]